1. Field of the Invention
The present invention generally relates to electrochemical cells and more particularly to cuprous chloride-containing cathodes for electrochemical cells.
2. Prior Art
Silver chloride cathodes have a variety of uses, mainly in electrochemical cells employing a sea water as the electrolyte and a dissimilar anode, such as a magnesium anode. Silver chloride cathodes are relatively easily fabricated because silver chloride can be easily rolled, cast or extruded through a die while hot to form a self-supporting, integral structure. However, silver chloride is very expensive and, accordingly, attempts have been made to substitute less expensive materials for the same in fabricating suitable cathodes for such purposes.
It would also be desirable to provide an electrode capable of producing more power from the same volume or weight of material than is possible with a silver chloride electrode. Copper chloride has been substituted for silver chloride as the cathode material in a number of electrochemical cells such as those which have been used in certain buoys and radiosondes. Cuprous chloride, when used as a cathode in association with a magnesium anode and sea water as the electrolyte, normally produces 1.3-1.4 volts at a current density of about 200 ma/in.sup.2. When silver chloride is used in place of the cuprous chloride in such as cell, it normally produces 1.5 volts and has a flat discharge curve for most of the discharge.
Theoretically, cuprous chloride should deliver 50% more coulombs than the same weight of silver chloride but this, in practice, does not occur because of the direction reaction of water in the electrolyte with cuprous chloride. Moreover, certain portions of the cuprous chloride electrode may suffer passivation during manufacture of that electrode. Cuprous chloride, in contrast to silver chloride, deteriorates rapidly in the presence of humidity. Therefore, complete sealing of cuprous chloride cathodes has had to be resorted to, particularly where any length of storage of the cathode before use has been involved. Although cuprous chloride is about 1/10th the price of silver chloride, the extra work and precautions involved in processing and protecting cuprous chloride make it disadvantageous economywise for certain smaller battery sizes.
Cuprous chloride is normally obtained as white powder in its freshly prepared pure state but turns green and may cake if exposed to the atmosphere. Although cuprous chloride is essentially insoluble in water (solubility of about 60 parts per million in cold water), cupric chloride, one of the reaction products of cuprous chloride with the atmosphere, is very soluble in water. Soluble cupric chloride in an electrochemical cell will result in precipitation of copper on negative electrodes with unfortunate results.
One method of manufacturing cuprous chloride electrodes involves pressing the cuprous chloride powder onto a copper mesh screen to form a plate and wrapping the resultant plate with porous paper. The paper may be pressed or cemented on the surface of the plate. However, this electrode is inferior in voltage comeup time on activation and deteriorates rapidly when exposed to humidity.
Another method of producing a cuprous chloride electrode involves forming a paste of cuprous chloride particles, water or other solvents, and a binder such as carboxymethyl cellulose or vinyl chloride, and then shaping the paste into desired form and drying it at about 100.degree. C. The resulting electrode is usually in the form of a plate which is less prone to deteriorate on storage but has even poorer voltage come-up time on activation than does the pressed powder plate previously described.
Accordingly, there is still a need for a suitable substitute for silver chloride in electrodes, particularly cathodes for use in sea water cells. Such material should be resistant to deterioration during storage and should exhibit superior electrical properties in use. It also should be of inexpensive materials and be inexpensive to fabricate into the desired form.